Cartesian feedback, as it is now known in the communications industry, was first mentioned in the context of magnetic resonance in 1989 [C-N. Chen, D. I. Hoult, Biomedical Magnetic Resonance Technology, Adam Hilger, Bristol, 1989, P. 210]. This radio-frequency negative-feedback technique was briefly advocated there, without technical detail, as a cure for distortion in the magnetic resonance (MR) transmission chain.
W. A. Anderson, in U.S. Pat. No. 5,767,677 published Jun. 16, 1998 and entitled Suppression of radiation damping in NMR discloses a method for using feedback in NMR for the specific purpose of the compensation of radiation damping effects. Similar arrangements are disclosed in the papers by P. Broekaert and J. Jeener, Suppression of radiation damping in NMR in liquids by active electronic feedback, J. Magn. Reson. A113 (1995) 60-64 and in A. Louis-Joseph, D. Abergel and J. Lallemand, Neutralization of radiation damping by selective feedback on a 400 MHz spectrometer, J. Biomol. NMR 5 (1995) 212-216.
However, the present invention discloses that Cartesian feedback can be used in MR experiments in diverse ways to obtain specific effects of significant advantage.
Firstly, it is now well-established and clear that as static field strengths continue to increase, the use of assemblies of the electrical coils needed for MR transmission and reception (often termed “phased array coils”) will become more prevalent, particularly in magnetic resonance imaging (MRI). For signal reception, the use of such assemblies of coils, where each coil is appropriately tuned and noise-matched, has been shown unequivocally to yield over elongated volumes of interest a more homogenous spatial response function and/or improved signal-to-noise ratio (S/N). In addition, they may help to counteract propagation effects that are seen at high field strengths, e.g. field-focussing in head images at 8 T. (This is an especial hope during transmission where the coils are tuned and power-matched.) From their inception, however, it was clear that electromagnetic interactions, both direct and via the intermediary of the patient or sample, presented problems in both transmission and reception, as these interactions detuned the coils and introduced correlations between signal and noise voltages.
During signal reception, it is well-known that these interactions are typically tackled: a) By annulling nearest-neighbour reactive components, either by overlapping the coils or by the use of various bridges. (Despite the availability of venerable four-quadrant bridge designs that also remove the resistive components of mutual impedance, these are not usually employed as the resistive cancellation degrades signal-to-noise ratio (S/N) and increases noise correlation.) b) By utilising, in addition, the transformation properties of each coil's tuning and matching network, in conjunction with low input impedance pre-amplifiers, to present a large impedance in series with the coil. This blocks residual resistive current flow in nearest neighbours and is also effective against the smaller combined (resistive and reactive) induced voltages in distant neighbours. The deleterious effects of the interactions are thereby rendered negligible and the coils are effectively decoupled. Note that decoupling methods external to the coils have also been described.
During transmission, an equivalent strategy is to retain the cancellation of nearest-neighbour reactive coupling but to mismatch grossly each transmitter so that each effectively presents a high impedance in series with its coil. However, the loss of efficiency that would be required to produce an adequate decoupling in this manner is intolerable, so other means are needed effectively to increase the transmitter source impedance. The solution we present here is the use of Cartesian feedback.
Secondly, there are numerous defects in, and deficiencies of, magnetic resonance instrumentation, arguably the most serious being that a spectrometer or imager is uncalibrated in an absolute manner. For example, when transmitting, a change of patient in MRI or of the sample in spectroscopy (e.g. non-conducting to conducting, polar to non-polar, etc.) often necessitates a recalibration of pulse power or length to maintain constant excitation of the atomic nuclei. This is because an electrically conducting or dielectrically-lossy sample absorbs power and hence reduces the coil current. Recalibration (albeit less) is still needed even if the coil (or coils) has been re-tuned and re-matched—and this may not even be possible if automatic sample-changing is employed. Concomitantly, upon reception, even if different samples, or the sampled volumes of different patients under observation, contain the same number of nuclei, and even if the excitation has been maintained constant in some manner, the amplitudes of the received free induction decay (FID) signals, as presented to the receiver by the tuning and matching circuitry, may change. With ideal instruments, such changes would not occur. Thus, to take but two examples among many, there would be no need to worry that the swelling of a perfused heart would change the pulse length and the signal strength of some metabolite of interest, or that the breathing of a patient in an imaging experiment would do likewise. A concise way of interpreting this defect is to consider that the effective electrical gains of the transmission and reception chains in magnetic resonance experiments are variable in a manner that depends on the sample or patient.
While we have essentially noted above that change of sample characteristics can change the electrical characteristics of transmission and reception coils, there are also other factors which can do the same. For example, coil capacitance and inductance can be modulated by movement and vibration, the latter causing extra “1/f” noise about a resonance signal in a magnetic resonance spectrum and “t1” noise in two dimensional spectra, via phase modulation. Such vibration can come from air flow, from spinning the sample, from imaging gradient noise, etc. Spinning the sample in high resolution or magic angle experiments can also directly modulate the coil tuning if there is any heterogeneity or asymmetry in the electric susceptibility of the sample. This causes extra spinning sidebands about spectral lines. If more than one coil at the same frequency is used, these sensitivities can increase greatly due to coupling. Examples are high resolution quadrature coils, coils in rotating frame imaging experiments, the phased-array coils mentioned above, etc. While “paddles” or capacitor-resistor leakage bridges can cancel both reactive and resistive interactions, the sensitivity of the balance still renders such solutions susceptible to the factors mentioned above. Change of temperature also affects the electrical characteristics of coils and transmission and reception chains, causing excitations and received signal strengths to drift in amplitude and phase. Now during signal reception, the use of pre-amplifiers with low input impedance can once again ameliorate these problems. Concomitantly, mismatching the transmitter can, in theory, do likewise during transmission. However, as we have seen, the loss of efficiency is unacceptable.
Turning to other ills, crossed diodes in radio-frequency transmit/receive switch circuits cause considerable non-linearity at low pulse powers and as a result, can greatly distort selective or shaped pulses. PIN diode circuits help considerably here, but even they are not distortion-free and their switching times are often excessive. Further, the design of direct current paths that do not compromise the performance of some imaging probes is sometimes problematic. Finally, high power transmitters are renowned for changes of power and phase during a pulse as components rapidly heat, and with poor designs, power supply voltages droop. As noted in Chen and Hoult [op. cit.], pulses are then distorted in amplitude and phase and of course, ambient temperature changes have their destructive role to play here too. Finally, as already noted, radiation damping is a problem of some importance in certain experiments. The use of Cartesian feedback is presented as a solution to all the above problems.